Underground cables, such as power, telephone and cable TV lines (CATV), are in wide use and typically include one or more wires encased in a metallic sheath covered by insulating material. Although the lines are manufactured to endure the harshness of underground burial, they do not always escape physical damage that may allow moisture intrusion through the insulation cover, thereby causing a short to ground.
A fault detection system described in U.S. Pat. No. 3,299,351 to Williams applies to an underground cable, a low frequency AC test signal and a reference signal at one-half the frequency of the test signal. Differential probes detect the signals when they leak to ground from a cable fault, and circuitry attached to the probes doubles the reference signal's frequency. Because the phase of the leaked test signal, but not the phase of the doubled reference signal, changes depending on the relative position of the probes and the fault, a comparison of the reference signal phase to the phase of the test signal indicates the direction to the fault.
A fault detection circuit disclosed in U.S. Pat. No. 3,991,363 to Lathrop also relies on a phase comparison between test and reference signals applied to an underground cable. The test signal is carried by a high frequency RF carrier. The high frequency reference signal radiates from the earth and is detected by above ground antennas. Differential probes detect the lower frequency test signal when it leaks to ground from a cable fault. A phase reversal takes place between these two signals at the fault. Thus, comparison of the signal phases indicates the fault's direction with respect to the receiver.